Over the years, various techniques and devices have been developed to help a person accurately aim a weapon such as a rifle. One common approach is to mount a sight or scope on the weapon. A person then uses the sight or scope to view an intended target in association with a reticle, often with a degree of magnification. Although existing weapon sights have been generally adequate for their intended purposes, they have not been satisfactory in all respects.
For example, after mounting a sight on a rifle, there is always some pointing error for a specific target distance that must be “zeroed” out in order to shoot accurately. In other words, for the target distance in question, an adjustment needs to be made so that the reticle cross-hairs of the sight will be centered on the point where the bullet will strike at that distance. One known approach is to use an external adjustment mechanism that adjusts the position of the entire sight in elevation and azimuth with respect to its mount on the rifle. A different known approach is to move an optical element internally within the sight, in order to readjust the effective pointing direction of the sight.
The traditional approach for internal zeroing is to move the optical erecting element within the sight. This essentially moves the secondary image plane so as to correct for the zeroing error. Some sights that use this approach are relatively long, because the objective image is reimaged by the erecting element to a conjugate plane, which is then viewed through the eyepiece. Shorter and more compact sights use a prism to erect the image. The prism can be internally moved with respect to other optical components, in order to displace the image at the reticle plane.
Although known techniques for internal zeroing have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects. One consideration is that the known techniques for internal zeroing will realign the aim point of the rifle, but also involve a degradation of the image quality. For example, moving an erecting prism within the light path relative to other optical components can produce axial coma and astigmatism that are associated with a loss of resolution for the axial and off-axis fields. This limits the effective zeroing range for this approach to a fraction of a degree.